PHP is still a broken language - it kept on evolving without any formal specifications until recently; hence the reason for the mess. If they do fix, it won't be PHP anymore. Either way, I believe it will be mostly extinct in 20 years time, here's my reasoning.
Wikipedia:
Early PHP was not intended to be a new programming language, and grew organically, with Lerdorf noting in retrospect: "I don’t know how to stop it, there was never any intent to write a programming language […] I have absolutely no idea how to write a programming language, I just kept adding the next logical step on the way."[14] A development team began to form and, after months of work and beta testing, officially released PHP/FI 2 in November 1997.
The fact that PHP was not originally designed but instead was developed organically has led to inconsistent naming of functions and inconsistent ordering of their parameters.[15] In some cases, the function names were chosen to match the lower-level libraries which PHP was "wrapping",[16] while in some very early versions of PHP the length of the function names was used internally as a hash function, so names were chosen to improve the distribution of hash values.[17]
What PHP has going for it is its ease of use which means many developers jumped onto the bandwagon even people who've never done any development. I believe JavaScript is now becoming that go-to language or even Python and given enough time, shared hosting companies will start to offer NodeJS alongside Apache and PHP will start to fade.
PHP's initial claim to fame was in its acronym, Personal Home Page which later changed to the recursive acronym PHP Hypertext Preprocessor. The age of needing a Hypertext Preprocessor is long gone, browsers are powerful enough to do all that stuff now with JavaScript (so normal HTML + JS + CSS is the future of static pages instead of PHP) and blogs / forums as well as personal pages are fading fast from the web thanks to social networks - blogs and forums used to be all PHP.
Looking at this graph, PHP's popularity has been in steady decline over the last 10 years: tiobe.com/index.php/content/paperinfo/tpci/PHP.ht…
Compared to Java which is a well designed language, Java's popularity has shot through the roof in the last few years and if you go by what has happened in history so far, Python (which is also a well designed language) is much more likely to become a successor to PHP as a scripting language as it already almost replaced perl when it came along. tiobe.com/index.php/content/paperinfo/tpci/index.…
You can argue that Zend and HHVM and other variants are developed by large companies - actually those are just stop gaps to fill the holes that PHP could never fill.
So my verdict is that in 20 years, PHP will be one of the legends that everyone knows about, but nobody uses anymore. That's obviously assuming we'll still be using websites in 20 years time and normal computers, if we don't, it could spell the doom for many more languages.
Edit: Abandoned websites will always be around, here are some samples from around 1995:
warnerbros.com/archive/spacejam/movie/jam.htm
home.mcom.com/home/welcome.html
youvegotmail.warnerbros.com/cmp/0frameset.html
Jan Vladimir Mostert
Idea Incubator